Communication method and system for the transmission of time-driven and event-driven Ethernet messages

ABSTRACT

The invention relates to a communication method for transmitting Ethernet messages in a distributed real-time system in which a plurality of network node computers, e.g. four network node computers ( 111, 112, 113, 114 ), each comprising at least one communication controller ( 121, 122, 123, 124 ), are linked via a communication system comprising one or more communication channels ( 109 ), one or more intelligent star couplers ( 101, 102 ) being disposed in each communication channel. According to the invention, a distinction is made between conventional Ethernet messages (ET messages) and time-triggered Ethernet messages (TT messages), the TT messages being transported with an a priori known constant delay time (Δ) between transmitter and receiver, and, when there is a time conflict between ET and TT messages, the transport of the ET message that is in conflict being delayed or aborted in order to be able to transport the TT message with the constant delay time (Δ). Furthermore, the invention relates to a corresponding communication system and a star coupler for such a communication system.

The invention relates to a communication method for transmittingEthernet messages in a distributed real-time system in which a pluralityof network node computers, e.g. four network node computers, eachcomprising at least one communication controller, are linked via acommunication system comprising one or more communication channels, oneor more intelligent star couplers being disposed in each communicationchannel.

Furthermore, the invention relates to a star controller for acommunication system for the transmission of Ethernet messages in adistributed real-time system including a plurality of network nodecomputers, e.g. four network node computers, each of which comprises atleast one communication controller, the communication system comprisingone or more communication channels via which the network node computersare connected to each other and one or more intelligent star couplersbeing disposed in each communication channel.

In the text to follow, reference is made to the literature indicatedbelow:

[1] U.S. Pat. No. 5,694,542 issued on Dec. 12, 1989: A Loosely CoupledDistributed Computer System with Node Synchronization for Precision inReal Time.

[2] EP 0 658 257 of Dec. 18, 1996: Communication Unit and Method for theTransmission of Messages.

[3] U.S. Pat. No. 5,887,143 issues on Mar. 23, 1999: Time-TriggeredCommunication Control Unit and Communication.

[4] AT 407 582 of Jun. 15, 2000: Message Distribution Unit havingIntegrated Guardian for the Prevention of Babbling Idiot Errors.

[5] AT 408 383 of Mar. 15, 2001: Method and Communication Control unitfor the Multi-master Clock Synchronization in a Distributed Real-timeComputer System.

[6] Austrian Patent Application 1723/2001 of Oct. 10, 2000: Method forthe Toleration of Slightly-off-specification Errors in a Distributed,Fault-tolerant, Real-time System.

[7] Austrian Patent Application 429/2001 of Mar. 19, 2001: CommunicationMethod for the Establishment of Event Channels in a Time-triggeredCommunication System.

[8] IEEE Ethernet Standard 802.3 at URL:HTTP:/standards.ieee.org

[9] Kopetz, H. (1997). Real-Time Systems, Design Principles forDistributed Embedded Applications; ISBN: 0-7923-9894-7. Boston. KluwerAcademic Publishers

[10] Sharon, O., Spratt, M., “A CSMA/CD compatible MAC for real-timetransmission based on varying collision intervals”. In: INFCOM'98.Seventh Annual Meeting Joint Conference of the IEEE Computer andCommunications Societies. Proceedings. IEEE, Volume: 3, 1998, pp.1265-1272 vol. 3.

In the past twenty years, the IEEE Ethernet Standard 802.3 [8] hasgained such wide acceptance that, because of the mass market forEthernet controllers in the personal computer area, the costs forEthernet-based communication systems have dropped very sharply. Forthese cost reasons, Ethernet is also used increasingly in real-time dataprocessing, although the existing Ethernet protocol possesses no goodreal-time properties, such as minimal Jitter.

A CSMS/CD system is known from [10] in which messages are subdividedinto such low and high priority, the message with high priority beinggiven the preference when there is a conflict between two messages.

However, the real-time properties of the Ethernet protocol cannot besubstantially improved with the procedure proposed here by itself.

One object of the invention is to enable the transmission of Ethernetmessages with good real-time properties.

This objective is achieved using a method of the type mentioned at theoutset, in that a distinction is made according to the invention betweenconventional Ethernet messages (ET messages) and time-triggered Ethernetmessages (TT messages), the TT messages are transported with an a prioriknown constant delay time between transmitter and receiver, and, whenthere is a time conflict between ET and TT messages, the transport ofthe ET message that is in conflict is delayed or aborted in order to beable to transport the TT message with the constant delay time.

Furthermore, the objective mentioned at the outset is achieved using astar controller mentioned at the outset that is set up according to theinvention to distinguish between conventional Ethernet messages (ETmessages) and time-triggered Ethernet messages (TT messages), totransport the IT messages having an a priori known constant delay timebetween transmitter and receiver, whereby, when there is a time conflictbetween ET and TT messages, the transport of the ET message that is inconflict is delayed or aborted in order to be able to transport the TTmessage with the constant delay time.

In contrast to the “non pre-emptive” solution disclosed in [10], in thepresent invention it is not necessary to wait for the end of thetransmission of a message having low priority; rather, the low prioritymessage is aborted in order to be able to transmit the high-prioritymessage (“pre-emptive”). As a result it is also not necessary to wait onthe maximum run time of messages having low priority, and the constantlatency can thus also be kept short.

By guaranteeing a constant delay time, it is possible to achieve a highcontrol engineering precision. The constant delay time is therefore ofspecial significance for the reason that, as is known from the theory ofclock synchronization, the variability of the delay time (this is thedifference between the maximum and minimum delay times) makes theprecision of the clock synchronization worse. An a priori known constantdelay time can be considered in the clock synchronization algorithm andtherefore has no effect on the precision of the clock synchronization.An imprecise clock synchronization brings about a poor time basisbecause the granularity of the global time must be greater than theprecision of the clock synchronization. A coarse granularity of theclocks brings about an imprecise time resolution of events. Furthermore,the variability of the delay time also determines the precision of thesynchronization of distributed actions in a distributed computer system.

The present invention makes it possible to improve substantially thereal-time properties of a communication system. This new communicationsystem supports the parallel operation of event-triggered andtime-triggered Ethernet messages in a single communication system. Inthe following, the classical Ethernet messages are characterized as ET(event-triggered) messages and the time-triggered Ethernet messages asTT (time-triggered) messages. The TT messages have a constant delay timeand minimal jitter.

The following significant economic advantages are produced by theinvention. The minimal jitter of the TT-messages enables the developmentof closed-loop control circuits of high control engineering quality. TheTT messages enable the development of a global time with good precision.Global time supports the generation of precise local time stamps in thedata acquisition and makes it possible to improve the temporalspecification of the interfaces. Moreover, conventional Ethernetcontrollers may be used without modification.

The method of the invention can be produced especially simply if theconstant delay time is selected in such a manner that the outputchannels of the star coupler may be cleared for the transport of the TTmessage to be received.

In one embodiment, there is an indication in a designated field of themessage of whether the message is a TT message or is an ET message.

Furthermore, an optional time field that indicates the transmissioninstant of the message may be contained within a TT message.

In this context it is advantageous if it is already established by an apriori planning that a time interval of at least the constant delay timeis observed between the Transport of two TT messages.

Furthermore, the aforementioned objective is achieved using a systemmentioned at the outset that is set up according to the invention todistinguish between conventional Ethernet messages Ethernet (ETmessages) and time-triggered Ethernet messages (TT messages) and totransport the TT messages with an a priori known constant delay timebetween transmitter and receiver, whereby, when there is a time conflictbetween ET and TT messages, the transport of the ET message that is inconflict is delayed or aborted in order to be able to transport the TTmessage with the constant delay time.

As already mentioned above, it is advantageous if the constant delaytime is selected in such a manner that the output channels of the starcoupler can be cleared within this delay time for the transport of theincoming TT message.

Moreover, it may be provided that there is an indication in a designatedfield of the message of whether the message is a TT message or an ETmessage.

Furthermore, an optional time field that indicates the transmission timeof the message may be contained within a TT message.

In this context it is advantageous if it is already established by an apriori planning that a time interval of at least the constant delay timeis observed between the Transport of two TT messages.

In a concrete embodiment of the communication systems, provision is madethat the instants at which incoming messages are TT messages areindicated to the star coupler via a configuration message.

In this context, the star coupler distinguishes between TT messages andET messages and transports the TT messages with an a priori knownconstant delay time, and, when there is a time conflict between ET andTT messages, it aborts the transport or the ET message that is inconflict in order to be able to transport the TT message with theconstant delay time.

Provision is then made that after the on-time transmission of a TTmessage the star coupler re-transmits the ET message that was inconflict and was aborted. Furthermore, it may be provided that the starcoupler synchronizes its local clock using the time field containedwithin a TT message.

In this context, it is especially beneficial if the star coupler in afault-tolerant manner synchronizes its local clock using the time fieldscontained within a plurality of TT messages. In addition, it may beprovided that the star coupler is connected to the replicated starcouplers within a cluster of network node computers via a dedicatedunidirectional channel on which all TT messages that the star couplertransports are output.

In addition, it is also still possible that the star coupler, on thebasis of its local time, checks whether the TT message arrives within ana priori known time window around the instant of transmission, and thestar coupler, when it receives a TT message early or late, garbles themessage in such a manner that all correct receivers detect the messageas faulty. The star coupler decodes each TT message and re-codes it onthe basis of its local timing module.

The star coupler reads one or more selected fields of TT messages andchecks during the delay time whether the content of these fieldscorresponds to known criteria that were communicated a priori to thestar coupler via a configuration message. If it does not correspond, themessage is garbled in such a manner that all correct receivers detectthe message as faulty. Moreover, provision is also made according to theinvention that the communication controller synchronizes its local clockusing the time field contained within a TT message.

The communication controller in a fault-tolerant manner synchronizes itslocal clock using the time fields contained within a plurality of TTmessages. Moreover, the communication controller autonomously transmitsa TT message accepted by an application executed on a network nodecomputer as soon as the transmission instant indicated in the time fieldin the message is reached.

Moreover, provision is made that the communication controllerdistinguishes between ET and TT messages, and the communicationcontroller provides the ET messages to the local application softwarecorresponding to the event semantics, a new message being placed in awaiting queue from which it is read in a consumptive manner by theapplication software, and the communication controller provides TTmessages to the local application software corresponding to the statussemantics, a new message replacing the old one and the reading by thelocal application software occurring in a non-consumptive manner.

Finally, the communication controller has two or more communicationchannels on which identical copies of a TT message are provided, and theone communication operation is considered successful if a valid TTmessage is received on time on at least one of these redundant channels.

The invention is explained in below in reference to the drawing. Shownare:

FIG. 1 the structure of a distributed computer system with a starcoupler,

FIG. 2 the structure of a distributed computer system with two starcouplers,

FIG. 3 the structure of a standardized normal Ethernet message,

FIG. 4 the structure of a standardized expanded Ethernet message,

FIG. 5 the structure of a TT Ethernet message, and

FIG. 6 the bit arrays of a TT parameter field of the TT Ethernetmessage.

In the next section, an embodiment of the new method is shown by anexample with four network node computers, which are connected via tworeplicated star couplers.

FIG. 1 shows a distributed computer system with a star coupler. Itcomprises four network node computers 111, 112, 113, and 114, each ofwhich has a communication controller 121, 122, 123, and 124 with onebidirectional communication channel connection each, and which areconnected via a communication system comprising a communication channel109. Located in this communication channel is an intelligent starcoupler 101 for the central control of communication. The star coupler101 can be initialized and observed via an optional, separatecommunication channel 141.

FIG. 2 shows a distributed fault-tolerant computer system having twostar couplers. It is comprised of four network node computers 111, 112,113 and 114, each of which has a communication controller 121, 122, 123and 124 with two bi-directional communication channel connections each.Each of these communication channel connections is connected to anintelligent star coupler 101 and 102, which carry out the centralcontrol of communication. Star coupler 101 is able to transmit itsmessages via channel 151 to star coupler 102 and can be initialized andobserved via separate communication channel 141. Star coupler 102 isable to transmit its messages via channel 152 to star coupler 101 andcan be initialized and observed via separate communication channel 142.

FIG. 3 shows the structure of a normal Ethernet message standardizedaccording to [8]. Situated after a preamble 301 with a length of 7 bytesare start delimiter field 302, target address 303, transmitter address304, message length or type of message 307, variable data field 310,optional PAD field 311, with which short messages are extended, andframe-check sequence 312. FIG. 4 shows the structure of an expandedEthernet message standardized according to [8]. In addition to thefields described in FIG. 3, an identifier for the expanded message islocated in field 305 and a tag-type field is located in field 306. Inthis tag-type field, the user may determine the priority of a message.The highest priority may be used according to the present invention forthe identification of a TT message. Such an identifier conforms to theEthernet standard [8]. It should be pointed out that in the Ethernetstandard the code allowance for field 305 is not yet fully utilized, andtherefore this field could also used for the identification of a TTmessage. FIG. 5 shows the structure of a TT Ethernet message. Inaddition to the fields described in FIG. 4, in field 308 a TT parameterfield is introduced and in field 309 the optional transmission instantof the TT message is indicated. A standardized Ethernet controlleravailable on the market reads user-specific data fields in fields 308and 309. In TT parameter field 308 is information pertaining to thestructure and the type of the TT message.

FIG. 6 shows the contents of the bit arrays of TT parameter field 308.If the bit is set in field 601 (low-order bit), this means that thetransmission instant in field 309 is contained in the TT message. If thebit is set in field 602, this means that the message comes from atransmitter having a precise clock time and may be used for clocksynchronization.

If a network node computer, e.g. 111, wants to transmit a TT message, itsets the code for a TT message in message field 306 and transmits themessage. Alternatively, the application software running on a networknode computer can set bit 601 in the message and write the desiredtransmission point in field 309 of the message. The start oftransmission can then autonomously occur precisely at establishedtransmission instant 309 via an Ethernet communication controller. Ifthe transmitter sets the bit of the message, then the message containsan especially precise time indication, which can be used for the clocksynchronization of the other controllers.

The star coupler analyzes an incoming message and using field 306determines whether a TT message or an ET message is arriving. In thecase of a TT message, the star coupler determines the desired outputchannel on the basis of field 303, e.g. at node 114 in FIG. 1. If an ETmessage is sent directly on this channel, then the star coupler abortsthis transmission operation immediately and clears the channel to node114 within a known constant delay time Δ for the further transport ofthe TT message that is just arriving. Delay time Δ must be selected tobe long enough that in each case the output channel can be clearedwithin this delay time Δ for the transport of the TT message. Within thecontext of an a priori planning of the TT communication, it must beensured that the interval between consecutive TT messages is greaterthan delay time Δ. In the individual case, the star coupler preciselyobserves this constant delay time Δ between the beginning of thereception of a TT message and the beginning of the transmission of a TTmessage. If the star coupler has aborted the transport of an ET messagethat is in conflict, it can re-transmit the aborted ET message after theon-time transport of the TT message. The star coupler can also take onguardian functions, as are described in [4], in order to detect andisolate faulty messages and thereby prevent the propagation of errors.If a TT message in field 309 obtains the transmission instant, then thestar coupler can check whether the message arrives according to [6]within a known tolerance interval around the transmission instant andreject the message if this is not the case. Alternatively, the inputchannels on which TT messages are expected and at what instants can becommunicated to the star coupler via channel 141 using a configurationmessage transmitted a priori. This information redundancy in afault-tolerant system prevents a faulty computer node from entering anincorrect transmission time. Because the star coupler codes the messageon the basis of its own oscillator and its own power supply on theoutput, the forwarding of an SOS fault from the transmitter to thereceiver [4] is stopped. The star coupler can initially synchronize itslocal clock in that it measures the beginning of the reception of a TTmessage and sets its clock in such a manner that at this receptioninstant it would have accepted the value of global time 309, which iscontained in the message [5].

A continuous fault-tolerant clock synchronization may be realized asfollows: with each synchronization message marked in field 602, the starcoupler determines the interval between the reception instant of thesynchronization message measured with its local clock and thetransmission instant [5] contained in field 309 of the message.

This interval is a measure for the deviation of the clock of thereceiver from the clock of the transmitter. If a number of such messagesis present, then it is possible for the correction factor for the clockof the star coupler to be calculated using a known fault-tolerantsynchronization algorithm, as described in [9] on p. 61. Such afault-tolerant synchronization method may also be realized in thehardware of the star coupler [1]. In a fault-tolerant system [2, 3], inwhich replicated communication channels corresponding to FIG. 2 arepresent, each star coupler can send all TT messages via a dedicatedconnection channel (151 for star coupler 101 and 152 for star coupler102) to the other star coupler, so that the latter may also synchronizeits clock if no message arrives at its own input. In a fault-tolerantsystem, the star coupler, within delay time Δ, can check the content ofdata field 310 of the message corresponding to criteria reported to itvia a configuration message in order to detect data fields of thetransmitter. A message detected as faulty is not forwarded by the starcoupler.

If a receiving communication controller finds the transmission instantin field 309 of the arriving TT message, it can synchronize its localclock, in that it measures the beginning of the reception of the messageand its clock is set in such a manner that at this reception instant itwould have accepted the value of global time 309 contained in themessage plus constant delay interval Δ caused by the star coupler [5]. Acontinuous fault-tolerant clock synchronization can be realized asfollows: with each synchronization message marked in field 602, thecommunication controller determines the interval between the receptioninstant of the synchronization message measured with its local clock andthe transmission instant contained in field 309. It shortens thisinterval by the known delay interval Δ of the star coupler. Thisshortened interval is a measure for the deviation of the clock of thereceiver from the clock of the transmitter. If a number of such messagesis present, then it is possible, using a known fault-tolerantsynchronization algorithm, as described in [9] on p. 61, for thecorrection factor for the clock of the star coupler to be calculated.Such a fault-tolerant synchronization method may also be realized in thehardware of the receiving communication controller [1]. If theapplication software of a computer node, e.g. 111, enters the intendedtransmission instant of the message in message field 309, an expandedcommunication controller according to the invention can autonomously setup the beginning of the transmission [2,3] at precisely the correcttransmission instant. At the interface between the receivedcommunication controller, e.g. 121, and the application software, anexpanded communication controller can provide ET messages and TTmessages differently. ET messages normally contain information aboutevents and must be processed according to the event semantics [7]. Theevent semantics require that arriving messages be temporarily stored ina waiting queue and be transferred exactly one time to the user process.TT messages normally contain status data that can be provided inaccordance with the status semantics in a common memory. The receptionof a new TT message overwrites the memory value of an older TT messageby the same name. The receiving process reads status data in anon-consumptive manner. In a fault-tolerant system that provides aplurality of multiple independent communication channels, e.g. via twochannels as in FIG. 2, messages are sent in a replicated manner. In sucha system, the communication is successful if at least one of thereplicated message copies arrives at the receiver.

Finally, it should be noted that the previously described concreteimplementation of the integration of time-triggered and event-triggeredmessages in the Ethernet represents only one of many possibleimplementation variants of the invention.

For example, it is possible to derive the decision of whether a messagearriving at the star coupler is a TT message not from the messagecontent in field 306 or field 305, but from the instant of reception ofa message at the star coupler. In such a case, when and on which channela TT message is expected must be reported to the star coupler a priorivia a configuration message. The same is true for the communicationcontroller.

It is an essential characteristic of this invention that existingcommercially available Ethernet controllers can send and receivetime-triggered messages without modification.

1. A communication method for the transmission of Ethernet messages in adistributed real-time system in which a plurality of network nodecomputers, e.g. four network node computers (111, 112, 113, 114), eachof which comprises at least one communication controller (121,122, 123,124), are connected via a communication system that comprises one ormore communication channels (109), one or more intelligent star couplers(101, 102) being disposed in each communication channel, and adistinction being made between conventional Ethernet messages (ETmessages) and time-triggered messages (TT messages), and the TT messagesare transported with an a priori known constant delay time (Δ) betweentransmitter and receiver, wherein when there is a time conflict betweenET and TT messages, the transport of the ET message that is in conflictis aborted in order to be able to transport the TT message with theconstant delay time (Δ).
 2. The method as described in claim 1, whereinthe constant delay time (Δ) is selected in such a manner that withinthis delay time (Δ) the output channels of the star coupler (101, 102)for the transport of the incoming TT message can be cleared.
 3. Themethod as described in claim 1, wherein in an outstanding field of themessage a designation is made of whether the message is a TT or an ETmessage.
 4. The communications method as described in claim 1, whereinthe transmission instant of the message is indicated via an optionaltime field (309) contained in a TT message.
 5. The communication methodas described in claim 1, wherein it is determined via an a prioriplanning that a time interval of at least the constant delay time (Δ)between the transport of two TT messages is observed.
 6. A star couplerfor a communication system for the transmission of Ethernet messages ina distributed real-time system comprising a plurality of network nodecomputers, e.g. four network node computers (111, 112, 113, 114), eachof which comprises at least one communication controller (121, 122, 123,124), the communication system comprising one or more communicationchannels (109) via which the network node computers (111, 112, 113, 114)are connected to each other, and one or more intelligent star couplers(101, 102) being disposed within each communication channel, wherein itis equipped to distinguish between conventional Ethernet messages (ETmessages) and time-triggered Ethernet messages (TT messages) and totransport the TT messages having an a priori known constant delay time(Δ) between transporter and receiver, whereby, when there is a timeconflict between ET and TT messages, the ET message in conflict isaborted in order to be able to transport the TT message with theconstant delay time (Δ).
 7. The star coupler as described in claim 6,wherein the constant delay time (Δ) is selected in such a way thatwithin this delay time (Δ) the output channels of the star coupler (101,102) can be cleared for the transport of the incoming TT message.
 8. Thestar coupler as described in claim 6, wherein, in an indicated field ofthe message, a designation is made of whether the message is a TTmessage or an ET message.
 9. The star coupler as described in claim 6,wherein an optional time field (309) that indicates the transmissioninstant of the message is contained in a TT message.
 10. The starcoupler as described in claim 6, wherein it is determined via an apriori planning that a time interval of at least the constant delay time(Δ) between the transport of two TT messages is observed.
 11. The starcoupler as described in claim 6, wherein the instants at which incomingmessages are TT messages are reported to the star coupler via aconfiguration message.
 12. The star coupler as described in claim 6,wherein it distinguishes between TT messages and ET messages andtransports the TT messages and ET messages with an a priori knownconstant delay time (Δ) and, when there is a time conflict between ETand TT messages, aborts the transport of the ET message that is inconflict, in order to be able to transport the TT messages with theconstant delay time (Δ).
 13. The star coupler as described in claim 6,wherein it re-transmits the ET message that was in conflict and abortedafter the on-time transmission of a TT message.
 14. The star coupler asdescribed in claim 6, wherein star coupler synchronizes its local clockusing the time field (309) contained in a TT message.
 15. The starcoupler as described in claim 6, wherein, in a fault-tolerant manner, itsynchronizes its local clock using the time fields (309) containedwithin a plurality of TT messages.
 16. The star coupler as described inclaim 6, wherein it is connected via a dedicated unidirectional channel(151), on which all TT messages that are transported to the star couplerare output, to the replicated star couplers within a cluster of networknode computers.
 17. The star coupler as described in claim 6, wherein,for each TT message, it checks on the basis of its local time whetherthe TT message arrives within an a priori known time window around thetransmission instant (309) contained in the message and which, when a TTmessage arrives early or late, garbles the message in such a manner thatall correct receivers detect the message as faulty.
 18. The star coupleras described in claim 6, wherein it decodes each TT message and re-codesit on the basis of its local timing module.
 19. The star coupler asdescribed in claim 6, wherein it reads one or more selected fields of TTmessages and checks during the delay time (Δ) whether the content ofthese fields corresponds to known criteria that were communicated apriori to the star coupler via a configuration message and which, if itdoes not correspond, garbles the message in such a manner that allcorrect receivers detect the message as faulty.
 20. A communicationsystem for the transmission of Ethernet messages in a distributedreal-time system comprising a plurality of network node computers, e.g.four network node computers (111, 112, 113, 114), each of whichcomprises at least one communication controller (121, 122, 123, 124),the communication system comprising one or more communication channels(109) via which the network node computers (111, 112, 113, 114) areconnected to each other, and one or more intelligent star couplers(101,102) being disposed within each communication channel,characterized by a star coupler as described in claim
 6. 21. Thecommunication system as described in claim 20, wherein the communicationcontroller synchronizes its local clock using the time field containedin a TT message.
 22. The communication system as described in claim 20,wherein the communication controller in fault-tolerant mannersynchronizes its local clock using the time fields (309) containedwithin a plurality of TT messages.
 23. The communication system asdescribed in claim 20, wherein the communication controller autonomouslytransmits a TT message accepted by an application running on a networknode computer as soon as the transmission instant indicated in the timefield (309) in the message is reached.
 24. The communication system asdescribed in claim 20, wherein the communication controllerdistinguishes between ET and TT messages, and the communicationcontroller provides the ET messages to the local application softwarecorresponding to the event semantics, a new message being placed in awaiting queue from which it is read in a consumptive manner by theapplication software, and the communication controller provides TTmessages to the local application software corresponding to the statussemantics, a new message replacing the old one, and the reading by thelocal application software occurring in a non-consumptive manner. 25.The communication system as described in claim 20, wherein thecommunication controller has two or more communication channels on whichidentical copies of a TT message are provided and the one communicationoperation is considered successful if a valid TT message is received ontime on at least one of these redundant channels.